refactor: component files export only components; lint fails on a warning

The 43 react-refresh/only-export-components warnings the gate let through meant an edit to a
provider (or AnchoredPopover, Toaster, plexDetailUi, …) could not be hot-swapped: Fast Refresh
reloads a module that also exports hooks or helpers.

- Each provider's contexts, the types its hooks return and the hooks themselves live in a sibling
  context module (navigationContext.ts, channelsContext.ts, …); the provider file keeps the
  component. loadAccountFilters moved next to the feed filters' defaults (feedFiltersInit.ts),
  following navigationInit/playlistsInit.
- Helpers that sat in component files have their own modules: welcomeMode, downloadTitle,
  plexPlayable, channelSync, ui/anchoredPopoverHooks, ui/pageToolbarSlot, ui/pagerContent,
  ui/toastLevels, ui/overlayOrder, ui/tableAlign, lib/plexDetail; the cells of the channel columns
  and the registry's page wrappers are components files of their own (channelCells, modulePages).
- usePersistedTab, an alias of useAccountPersistedState, is gone; its callers use the original.

npm run lint is now eslint --max-warnings 0, so a warning fails the gate like an error does.

frontend/src/components/ConfigPanel.tsx

@@ -8,14 +8,14 @@ import { notify } from "../lib/notifications";
 import Tooltip from "./ui/Tooltip";
 import InlineNotice from "./ui/InlineNotice";
 import { useMe } from "../lib/useMe";
-import Tabs, { usePersistedTab } from "./ui/Tabs";
+import Tabs from "./ui/Tabs";
 import { LoadingState, StateMessage } from "./ui/QueryState";
 import { PageToolbar } from "./ui/PageShell";
 import { Switch } from "./ui/form";
 import SegmentedToggle from "./ui/SegmentedToggle";
 import FilterSelect from "./ui/FilterSelect";
 import { DraftSaveBar, type SaveState } from "./ui/DraftSaveBar";
-import { LS } from "../lib/storage";
+import { LS, useAccountPersistedState } from "../lib/storage";
 import { notifyActionError } from "../lib/actionErrors";
 
 // Admin Configuration page: edit DB-overridable operational settings (registry-driven from the
@@ -51,7 +51,7 @@ export default function ConfigPanel() {
   const [reseedToken, setReseedToken] = useState(0);
   // One tab per config group (persisted). Called before the loading guard so hook order is
   // stable; the active id is clamped to a real group at render time once data has loaded.
-  const [tab, setTab] = usePersistedTab(LS.configTab);
+  const [tab, setTab] = useAccountPersistedState(LS.configTab);
 
   // A stable signature of the config SHAPE — changes only when keys are added/removed, not when an
   // unchanged refetch produces a new baseline object. Re-seed the draft on a shape change (initial